During winter months, natural gas pipelines need to be protected to avoid freezing. At various locations along the pipeline, the pressurized natural gas passes through meters, orifice plates, or other in-line equipment which causes the natural gas to briefly increase in velocity. At these locations, condensate may form which freezes in cold weather. Stored methanol has long been used as a suitable fluid for injecting into a pipeline to prevent freezing. A small amount of added methanol prevents the formation of ice at these locations, and thus prevents pipeline freezing. A methanol injection pump may be powered by the natural gas pressure to inject a small quantity of methanol at a selected interval, e.g., 10 cc of methanol every 7 seconds, thereby preventing pipeline freezing.
Existing methanol injection systems may be deactivated in the Spring and reactivated in the Fall, since pipeline freezing is not a concern during warm weather. Existing systems undesirably utilize more methanol than required to prevent freezing, however, since an activated system continues to inject methanol even when the ambient temperature would not result in freezing. Since the methanol injection system is typically installed at remote locations which do not have available electric power, the power required to repeatedly activate and deactivate the system is of primary concern. Accordingly, existing systems remain continually on to inject methanol even when the ambient temperature will not result in freezing.
One proposed methanol injection system is the Style MS-1 system from Welker Engineering Co. in Sugar Land, Tex. This system utilizes a flow switch which is coupled to an orifice plate in the pipeline to sense the termination of natural gas flow through the pipeline. The system includes a solenoid powered by a 6-volt battery to control the flow of methanol from a storage tank into the pipeline. If the pipeline differential pressure across an orifice plate drops below a selected value, the solenoid valve activates a 4-way valve actuator, which in turn blocks the flow of methanol to the pipeline since natural gas is not being transmitted through the pipeline. As long as natural gas is flowing through the pipeline and the system is not manually deactivated for the Summer season, the metering pump is periodically activated by a timer to inject a selected quantity of methanol into the pipeline. This proposed system has had little if any commercial acceptance, primarily due to the power consumption of the monitoring system. As long as the monitoring system was activated, electrical power was required to maintain the solenoid valve in its proper position for controlling activation of the injection pump. As a practical matter, the high labor cost involved in continually replacing batteries often did not justify the benefit of saving methanol when flow through the pipeline was terminated.
The disadvantages of the prior art are overcome by the present invention, and an improved system and technique for periodically injecting a desired fluid into a pipeline is hereinafter disclosed. The techniques of the present invention are particularly well suited for controlling the injection of methanol into a natural gas pipeline to prevent freezing of the pipeline.